Laser processing systems in recent years are controlled by a numerical control device performing CNC (computer numeric control), and various functions can be realized by digital control of software. In these laser processing systems, a laser beam is obtained by discharge-exciting a laser medium by a laser power supply device or irradiating a laser medium with excitation light from an excitation light source. The laser beam is condensed to a small region on a workpiece by a condenser lens or the like to conduct a laser process.
In such a system, a digital signal from the numerical control device is converted to an analog signal, and the analog signal is supplied to a laser power supply device. The laser power supply device supplies electric energy to a laser medium in accordance with the analog signal. In such a manner, a laser beam having intensity as instructed is output at an instructed timing. From the numerical control device, a digital signal is transmitted in short cycles of 0.5 [ms] to 8 [ms]. The digital signal is converted to an analog signal, and the analog signal is transmitted to the laser power supply device. Further, the laser medium is excited, and a laser beam is emitted. In many cases, the operation from the digital signal transmission to output of the laser beam is completed within 0.1 [ms].
The procedure for laser cutting a workpiece by irradiating the workpiece with a laser beam is as follows. First, a process head for condensing a laser beam to the workpiece is brought close to a cutting start point in the workpiece. After the distance between the process head and the workpiece is optimized, piercing process is performed in the cutting start point on the workpiece. After completion of the piercing process, the irradiation position of the laser beam is moved in a desired direction and cutting of the workpiece is performed.
When an abnormality occurs in the laser process on the workpiece, output of the laser beam has to be stopped promptly in order to minimize damage to the workpiece. An abnormality in the laser process can be detected as abnormal radiation light from a process point in the workpiece irradiated with the laser beam. A technique of detecting an abnormality in the laser process in accordance with intensity of radiation light from the process point is known (for example, patent document 1).
The configuration of a conventional laser processing system will be described with reference to FIG. 1. According to a control signal from a processor 1021, an I/O unit 1024 drives a laser oscillator 1002. From the laser oscillator 1002, a pulse-shaped laser beam 1006 is emitted. The laser beam 1006 is reflected by a mirror 1003 and sent to a laser machine 1004.
The laser machine 1004 is provided with a table 1007 to which a workpiece 1008 is fixed and a process head 1005 for irradiating the workpiece 1008 with the laser beam 1006. The laser beam 1006 introduced to the process head 1005 is condensed near a process nozzle 1005a and the condensed light is emitted to the workpiece 1008. The laser machine 1004 is provided with servo motors 1009 and 1010 for moving the table 1007 in the X and Y axes. The laser machine 1004 is also provided with a servo motor 1011 for moving the process head 1005 in the vertical directions. The servo motors 1009, 1010, and 1011 are connected to servo amplifiers 1027, 1028, and 1029, respectively, and rotated in accordance with axis control signals from the processor 1021. An instruction to the laser machine 1004 is given via an input/output terminal 1025. To the process head 1005, a light amount detector 1012 is attached. The light amount detector 1012 detects radiation light generated at a process point (cutting point) via a lens (not shown) and outputs a signal of magnitude proportional to the detected light amount. The detection signal is amplified by an amplifier 1013 and the amplified signal is input to an A/D converter 1026 which converts the analog signal to a digital signal. An output of the A/D converter 1026 is supplied to the processor 1021.
During cutting of the workpiece, the amount of light detected by the light amount detector 1012 and generated at the process point to which the light is supplied via the amplifier 1013 and the A/D converter 1026 is read. The amount of the detected light is compared with a preset value of criterion of process abnormality. When the amount of the detected light is equal to or less than the criterion value, it is determined that no process abnormality has occurred. On the other hand, when the amount of the detected light is larger than the criterion value, the processor 1021 outputs an abnormality signal. By closing a laser beam blocking shutter (not shown) in the laser oscillator 1002 via the I/O unit 1024, the process is stopped. It takes at least some milliseconds to ten or longer milliseconds to close the laser beam blocking shutter on the basis of the light amount detected by the light amount detector 1012.
In the case of controlling an output of the laser on the basis of the light emission phenomenon at the process point by the laser irradiation like in the above-described conventional technique, there is a case that response speed to execute a process within 0.5 [ms] since an abnormality is detected until the process is stopped in order to prevent a workpiece from being damaged is required. However, as described above, in the conventional technique, it takes a few milliseconds to ten or longer milliseconds, since the light amount detector detects an abnormality until it is reflected in a laser output, which may result in workpiece being damaged.
Generally, laser cutting cannot be performed on an unprocessed workpiece under cutting process conditions from the beginning. Specifically, piercing process of forming a hole at a cutting start point has to be performed prior to the cutting process. First, the laser process head is brought close to the workpiece and is maintained at height optimum to the piercing process on the workpiece. Copper, brass, or aluminum alloy as a representative material of the workpiece has high reflectance to light including the infrared region used for a high-output laser. When the position of the light condensing point in the workpiece is not optimum, the laser beam reflected by the surface of the workpiece damages the mirror in a laser resonator, a fiber for excitation, and the like. Further, it may damage a mirror of a duct for beam transmission, a lens, and a fiber. Therefore, the light condensing point is positioned in an optimum position and, then, the laser beam has to be emitted.
Even when the workpiece is a flat plate, the workpiece may be, for example, slightly warped. In the laser processing system, the distance between the workpiece and the process nozzle is always measured by, for example, measuring capacitance between them. Feedback control is performed with the Z axis so that the distance between them is kept constant to make the distance between the process nozzle and the workpiece and the position of the light condensing point each having a narrow tolerance constant. At this time, it takes a few milliseconds to ten or more milliseconds since the distance between the process nozzle and the workpiece becomes optimum distance until irradiation of a laser beam is started, and causes an increase in time.
Further, the time required for the piercing process is not constant. That is, the process time varies even for the same workpiece due to various factors such as the properties, temperature, and the like of the surface of the workpiece. Therefore, in the laser cutting, the longest time to complete the piercing process is set as piercing process time. After the set process time elapses, the cutting process is performed. A laser process system of detecting completion of piercing process by a light amount detector is also known (for example, patent document 2). When the piercing process starts, light is emitted strongly at the process point. When the piercing process progresses and the hole penetrates the workpiece, light emission weakens. The light amount detector which detects light emission at the process time detects the change in strength and the end of the piercing process. After that, the laser output is stopped, and the cutting process is performed. In such a manner, the piercing process can be finished according to the length of the actual piercing process time. Therefore, the tact time can be shortened more than the case of performing the piercing process using the longest piercing time which is set as the piercing process time.
However, in the conventional method as described above, it takes a few milliseconds to ten or longer milliseconds since a signal from a light amount detector continues to be received until laser irradiation is actually stopped. The time is unignorable in the case of a laser process in which a process failure occurs when laser irradiation continues and heat input becomes excessive. Although the problem can be solved by endlessly shortening the interpolation cycle of the numerical control device, the computing power of the numerical control device is limited and the interpolation cycle cannot be shortened if various effective functions are not eliminated. Consequently, to shorten the interpolation cycle of the numerical control device, software and hardware generated on precondition that control is performed in predetermined interpolation cycles is largely changed. This is not a realistic solution.    Patent Document 1: JP-A-5-154676    Patent Document 2: JP-A-2-179377
The conventional laser processing system has a problem such that, in digital control by a numerical control device, since it takes too much time for a laser control instruction based on detection of an abnormality by a light amount sensor or detection of completion of a process until stop of a laser output.